1. Field of the Invention
The present invention is directed in general to magnetic resonance imaging (MRI) or magnetic resonance tomography (MRT), as employed in medicine for examining patients. The present invention is directed to a method as well as an apparatus for obtaining magnetic resonance images.
2. Description of the Prior Art
MRI/MRT is based on the physical phenomenon of nuclear magnetic resonance, and has been successfully employed in medicine and biophysics for almost two decades. In this examination modality, the examination subject (patient) is placed in a strong, constant magnetic field. The previously irregularly oriented nuclear spins in the subject are thereby aligned. Radio frequency energy then can excite these “ordered” spins to a specific oscillation. This oscillation generates the actual measured MR signal, which is picked up with suitable reception coils. By utilizing non-uniform magnetic fields, generated by gradient coils, the examination subject, and thus the MR signals emitted therefrom, can be spatially encoded in all three directions of a Cartesian coordinate system. In order to obtain data that are then processed to form the magnetic resonance image, a magnetic resonance imaging (tomography) apparatus is operated according to a sequence of radio frequency (RF) pulses and gradient pulses that are supplied to RF coils and gradient coils in a magnetic resonance scanner, in which the aforementioned strong, constant magnetic field is generated, and in which the patient is placed. Many such pulse sequences are known, and are generally designated with acronyms. One such known sequence is the FISP (fast imaging with steady precession) sequence. A version of the FISP sequence has been developed that is known as trueFISP, and is a promising tool for cardiovascular MRI, due to its inherently high blood signal and blood-myocardial contrast. In a trueFISP sequence, the magnetization in each repetition (TR) is recycled, thereby achieving a high blood signal-to-noise ratio and high blood-myocardial contrast-to-noise ratio.
Details of the known trueFISP sequence are available, for example, from U.S. Pat. Nos. 4,769,603 and 6,411,088.
A limitation of the trueFISP sequence is that is sensitive to off-resonance, which is directly proportional to the repetition time of the sequence. Another possible shortcoming may be due to imperfect excitation slice profiles that, when combined with blood flow, may cause image artifacts.